Previously, aluminum alloy plates have been used as supports for lithographic printing plates, and graining treatment is applied to the aluminum alloy plates for imparting adhesion with light-sensitive layers and water retention of non-image areas. A method of the graining treatment (i.e., the surface roughening treatment) hitherto known include mechanically graining processes such as ball graining and brush graining, electrochemically graining processes of electrolytic solutions mainly composed of hydrochloric acid or nitric acid, and electrochemically graining processes of etching a surface of aluminum alloy plate with acid solutions. In recent years, a combination of the electrochemically graining processes and other graining processes has come to constitute the mainstream, because the roughened surfaces obtained by the electrochemically graining processes have homogeneous pits (unevenness) and are excellent in printing performance.
However, even the electrochemically graining treatment is low in treating efficiency or produce the difference in the forming state of pits to cause failure to obtain homogeneous roughened surfaces in some cases, depending on the aluminum alloy plates used.
Then, for improving the efficiency of electrochemically graining treatment and equalizing roughened surfaces, the aluminum alloy composition has been variously studied. For example, JP-A-9-316582 (the term "JP-A" as used herein means an "unexamined published Japanese patent application publication") discloses an aluminum alloy plate containing 0.02% to 0.6% by weight of Fe, 0.03% to 0.1% by weight of Si, 0.04% to 0.1% by weight of Zn and 0.03% by weight or less of Cu, and having a concentration ratio of Zn to Fe (Zn/Fe) of 0.2 or more. Further, JP-A-9-279272 discloses an aluminum alloy plate containing 0.02% to 0.6% by weight of Fe, 0.03% to 0.15% by weight of Si, 0.005% to 0.05% by weight of Ti and 0.005% to 0.20% by weight of Ni, and an intermetallic compound of the above-mentioned elements having 20% to 30% by weight of Ni in addition to Al. Furthermore, JP-A-3-177528 discloses an aluminum alloy plate containing 0.03% to 0.30% by weight of Si, 0.1% to 0.5% by weight of Fe, 0.001% to 0.03% by weight of Cu, 0.005% to 0.002% by weight of Ga, 0.001% to 0.03% by weight of Ni and 0.002% to 0.05% by weight of Ti.
The alloy composition of surface layer portions (regions of from a surface to a depth of about several micron meters) of aluminum alloy plates has also been studied. For example, JP-A-10-204567 discloses an aluminum alloy plate containing 0.20% to 0.6% by weight of Fe, 0.03% to 0.15% by weight of Si and 0.005% to 0.05% by weight of Ti, wherein the Si concentration of a surface layer portion of from a surface to a depth of 3 .mu.m is 0.01% to 0.17% higher than that of the whole plate, and the surface layer portion contains 0.05% to 0.2% by weight of Si.
However, when the aluminum alloy compositions are specified as described above, it is necessary to add effective elements such as Zn and Ni in large amounts for obtaining the desired effects. Further, the necessity of adding plural kinds of elements causes increased cost. Furthermore, when the Si concentration of the surface layer portions of the aluminum alloy plates is increased, ink stains are liable to occur in non-image areas to raise a problem with regard to image quality.
Further, in the electrochemically graining treatment, the size, form and distribution of pits formed varies depending on electrolytic conditions such as supply electrical quantity, so that it is also necessary to strictly regulate and control the optimum electrolytic conditions.